There exist a variety of conventional laser liquid crystal markers, also called "transmission type liquid crystal mask markers". Generally speaking, a laser liquid crystal marker comprises a laser oscillator, a lens system for expanding, condensing or collimating a laser beam, a liquid crystal serving as a transmission type liquid crystal mask for arbitrary pattern display, one or two sets of X-Y deflectors for deflecting the optical path of the laser beam, a controller for performing a synchronized control of a separately provided workpiece feeder, etc. In such a construction, the controller causes the laser oscillator to emit a laser beam, which is applied to the liquid crystal. The laser beam transmitted through the liquid crystal is applied to the surface of a workpiece, whereby the pattern of the liquid crystal is printed on the surface of the workpiece.
In such a laser liquid crystal marker, the clearness in printing deteriorates with use. In the conventional laser liquid crystal markers, this deterioration has only been coped with empirically, that is, by methods that are not rational enough. The judgment as to whether or not the liquid crystal has deteriorated has been made by visually examining the printing conditions (the degree of clearness in printing, etc.) on the workpiece surface. With the current state of the art, any deterioration in the printing conditions has been coped with, for example, by appropriately increasing the laser output, cleaning the lens system, replacing the liquid crystal, adjusting the driving system of the X-Y deflectors or the workpiece feeder, or replacing the controller. Such measures cannot be regarded as appropriate in coping with the above deterioration.
According to a known printing method, a liquid crystal operation display section is provided in a part of the liquid crystal, and a detector is connected to this display section, which is irradiated with a beam of light different from that for printing to thereby detect changes in the response time of the liquid crystal due to changes in the ambient temperature; on the basis of this detection, the switching operation time for the laser is corrected by the controller, thereby realizing a clear printing on the surface of a workpiece (See, for example, Japanese Patent Laid-Open No. 64-11088). However, with the above described method, in which the operation time is corrected in correspondence with the changes in the response time of the liquid crystal, the degree of deterioration of the liquid crystal cannot be checked, nor can the liquid crystal itself be controlled for clear printing.
In another conventional technique, already proposed by the present applicant, there is provided a laser liquid crystal marker called a "YAG laser mask marker" (See Japanese Patent Laid-Open No. 5-42379), which is equipped with first and second X-Y deflectors arranged on either side of the liquid crystal. Separate still images are sequentially displayed through the liquid crystal. The first X-Y deflector causes a laser beam to scan each separate still image, and the second X-Y deflector causes the laser beam transmitted through the separate still images to be sequentially applied in deflective irradiation to the surface of a workpiece until the entire printing is completed, thereby realizing a high accuracy, high speed continuous printing. However, the above method has a problem in that the deterioration in the clearness in printing on the workpiece surface cannot be checked by a rational method, so that it is rather difficult to properly cope with the deterioration in clearness.